Dahlia Lithwick on Female Supreme Court Justices

It's a near certainty among Supreme Court watchers that President Obama will fill the bench's next vacancy with a woman. The current court's sole female member, Ruth Bader Ginsburg, has said she is "lonely" there, and even if she's not the next to step aside, that's still just two out of nine. Former Justice Sandra Day O'Connor, on learning in 2005 that John Roberts would take her place, declared him "good in every way, except he's not a woman." Americans seem to agree. In a CNN/USA Today/Gallup poll taken before Roberts was appointed, 80 percent of respondents said it was a good idea to replace O'Connor with a woman; 13 percent said it was "essential." And with many women claiming a large share of responsibility for Obama's victory over John McCain, the demand for a more gender-balanced court is stronger than ever.

But why does Obama owe us another woman justice? Is it simply a matter of appearance? Is it necessary for what political scientists call "social legitimacy"? Or is there something more fundamental that women bring to the bench—something about the way they decide cases—that makes the need so urgent?

Debate has raged for decades about whether there is something unique about women's jurisprudence. A 1986 study of O'Connor's opinions published by Prof. Suzanna Sherry, now at Vanderbilt University, saw evidence of a "feminine jurisprudence … quite unlike any other contemporary jurisprudence." The argument is often built on the groundbreaking work of psychologist Carol Gilligan, whose 1982 book, "In a Different Voice," claimed that female moral reasoning differs from that of males. Men, the theory goes, prefer their law with rigid rules, clear lines and neutral principles; women prefer to look at the totality of the circumstances and favor what Gilligan calls an "ethic of care" over an "ethic of rights." So, for example, feminists argue that O'Connor's preference for flexible standards regarding abortion (or for nonbelievers in cases about religion) reflect a softer, more "relational" approach to the law, while Justice Antonin Scalia's emphasis on unchanging rules and crisp legal principles is, fundamentally, a guy thing.

Empirical studies on gender and judging so far have been inconclusive. But in an award-winning 2008 paper titled "Untangling the Causal Effects of Sex on Judging," Washington University's Christina L. Boyd and Andrew D. Martin and Northwestern School of Law's Lee Epstein suggest that women judges really are different. Surveying sex-discrimination suits resolved by panels of judges in federal circuit courts between 1995 and 2002, they examined whether male and female judges rule alike, and whether the presence of a woman on a panel affects the behavior of her male colleagues. Here's what they found: male judges were 10 percent more likely to rule against alleged sex-discrimination victims, and male judges were "significantly more likely" to rule in their favor if a woman judge was on the panel.

Because Epstein, Boyd and Martin were only studying sex-discrimination cases, it's unclear whether their data would hold true in cases where gender was beside the point. Still, its intriguing that male judges rule differently when they're sharing the bench with a woman: it suggests female moral reasoning—if such a thing exists—might be contagious.

O'Connor, for her part, has been one of the most vocal detractors of the "different voice" theory of judging. She has said, "There is simply no empirical evidence that gender differences lead to discernible differences in rendering judgment." Paradoxically, O'Connor does believe that the personal experiences of diverse jurists impact the views of their colleagues. In her 2003 book, "The Majesty of the Law," O'Connor described how Justice Thurgood Marshall pushed his fellow justices to reevaluate their own moral truths and how his experiences with racial discrimination offered a window into that world for his white colleagues. It wasn't that Marshall wrote in a different voice. His real influence came in person—in his ability to paint for his colleagues a world they didn't know.

History shows that you can be the most empathetic and open-minded jurist in the world and still be a dolt about gender. It's why the liberal lion, Justice William Brennan, could write so expansively about equality and justice while still refusing to hire female law clerks. It's why Ginsburg was denied a clerkship with the legendary judge Learned Hand. (He refused to hire her because he liked to use salty language.)

When it comes time for Obama to appoint a new justice, he'll have a wealth of female talent to choose from. Potential candidates include appeals court judges like Diane Wood, Sonia Sotomayor and Kim McLane Wardlaw; his new solicitor general, Elena Kagan; gifted academics such as Stanford Law School's Kathleen Sullivan and Pamela Karlan; and private attorneys like Teresa Wynn Roseborough. Each has a different story, and nobody believes for a moment that they all think alike.

Still, beneath the formal legal reasoning of the Supreme Court, there are countless stories of casual influence: the female law clerks, the secretaries, the family members who, like Marshall, taught insulated justices how much they actually didn't know. Pushing diversity at the high court isn't just for show. The real point is to tell.